Reflection refers to a change in direction of light when they bounce off a substrate. Refraction is the change in the direction of light as it passes through a substrate.
So why should you know these terms? If you work with light modifiers, having an understanding of how they work will help you understand what kind of results you can expect from them. All modifiers used in photography fall under one of these two categories.
In physical space, a light wave will travel in a straight path, from the source of illumination, until one of two things happen. An object in the light path either absorbs it or the object deflects it. Typically all object tend to do both. As light encounters an object, some of the light's color spectrum gets absorbed and the rest gets deflected. What gets deflected is what allows us to see color. How an object deflects light is dependant on the physical structure, or surface makeup, of the object.